The present invention concerns a controller for a brushless D. C. motor, including a permanent magnet rotor and a two phase winding stator and, more particularly, to a brushless D. C. motor, including such a controller.
In the past, motor controllers have combined a simple logic circuit for producing two phase currents with a two phase bridge circuit to obtain bipolar switching. By closed loop and open loop control, i.e., asynchronous and synchronous control, a brushless D. C. motor can be controlled for a wide range of speed and torque.
Generally, however, the speed-torque characteristics of a motor controlled by asynchronous torque for speed control produces low torque at low speed and a wide fluctuation of speed according to load variation. Many methods were presented in the past to improve these characteristics, but the cost for implementing such methods was unsatisfactory.
In some prior controllers for D. C. brushless motors, the rotor speed is determined according to the stator winding current by the closed loop method, in which the position of the rotor is detected and a position signal is fed back to control the stator current so as to produce a desired torque at .pi./2 radians. It was possible to control the speed by varying the stator winding current because the rotating magnetic field produced by the stator coincides with the rotor speed. However, it is not possible for such a brushless motor to generate a constant torque for a wide range of the rotor speed because the speed-torque characteristics of such a brushless motor is almost identical to that of the conventional asynchronous motor.